1. Field of the Invention
The present invention relates generally to the field of telephony. More particularly, the present invention involves a simplified, smart data station termination ("DST") adapted for automatic alignment without the need for active equalizers.
2. Statement of the Problem
Data station termination ("DST") units are widely used to provide an interface between a conventional 4-wire telephone communications system and a conventional 4-wire data modem. For example, FIG. 2 shows communications between two data modems 21 and 22 over a typical telephone communications system. Both of the modems 21 and 22 have a DST 23 and 24, respectively, providing an interface with the local subscriber loop carrier ("SLC") 25 and 26. Each DST has the following ports: (1) receive-in; (2) receive-out; (3) transmit-in; and (4) transmit-out. The output signal from each modem 21 and 22 is at the 0 dBm level at the transmit-in port for each DST 23 and 24, as shown in FIG. 2. By convention, the DST produces a corresponding output signal at its transmit-out port at either +5 dB or -8 dB depending whether the DST is set for TLP (transmission level point) operation or DLP (data level point) operation. The signal is carried by voice-frequency cables to its SLC. Communications between the SLC's 25 and 26 typically occur over a T1 telephone line in digital format. By convention, the receive-in port of the DST accepts signal levels ranging from approximately 8 to -23 dBm DLP, or +5 to -10 dBm TLP. After installation, the receive channel of the DST should be aligned so that the signal level at the receive-out port is approximately -16 dBm.
Alignment of the DST receive channel is conventionally accomplished by using a switched access remote test system ("SARTS") located at the telephone central office to send a series of test tones at different frequencies to the receive-in port of the DST. A corresponding series of active equalizers are commonly used in the DST to provide the necessary gain at each of the test frequencies. These adjustments can either be done manually, or automatically if an intelligent DST is used. However, active equalizers add substantial complexity and expense to the DST, and also greatly complicate the alignment procedure.
A number of devices have been invented in the past incorporating smart DST technology. The INTELIPORT Data Station Termination, Model SDS9486, by Teltrend, Inc., 620 Stetson Avenue, St. Charles, Ill. 60174 is described in Teltrend Technical Publications 57-71-9486, Rev. B (February 1986). This device is an intelligent DST with an on-board microcomputer and precision oscillator which controls and generates test tones (1004 Hz, 2804 Hz, and 404 Hz) for alignment purposes, measures and calculates the amount of receive gain and equalization required at each frequency, and then automatically adjusts and verifies the adjustments for proper alignment. The unit also provides a loopback mode of operation.
The 4420GD RA Series 4Wire-to-4Wire Data Station Termination by Tellabs, Inc., 4951 Indiana Avenue, Lisle, Ill. 60532-1698 is described in Tellabs Technical Manual 76-814420GD (May 1987). This Tellabs unit is another example of an intelligent DST with an automatic alignment feature. Alignment takes place through a dialogue of tones at various frequencies sent from the central office ("CO") and returned by the DST. So that CO personnel can easily distinguish between tones sent to and tones returned by the DST, those tones returned by the DST are deliberately higher than the nominal CO test tone frequencies. Specifically, the DST returns tones of 1014, 2814, 414, and 1820 Hz in response to CO tones of 1004, 2804, 404, and 1804 Hz, respectively. This DST is capable of aligning only its receive-channel, or both its receive-channel and transmit-channel levels and equalizations. Again, equalization at each test frequency is provided.
3. Solution to the Problem
None of the prior art references show an intelligent DST that eliminates the need for active equalizers by using a single test tone from the SARTS for remote alignment of the DST.